Casteris the Caster-class Servant of Souichirou Kuzuki in the Fifth Holy
Grail War of Fate/stay night. She also acts as the Master of another
Servant, Assassin.
Caster's identity is Medea, an unfortunate princess who was labeled as a witch
in Greek mythology. She was the princess of Colchis who possessed the Golden
Fleece. Her father, King Aeetes of Colchis, excelled at magic, and as his
daughter, she was also practiced in such ways.